The invention relates to a method of testing tires, wherein before and after an alteration in the tire pressure a respective interferogram is produced of the tire surface using coherent radiation and the interferogram is converted into a modulo 2.sub..pi. image, which for its part is processed to obtain a gray value image and in the case of which from a comparison of the gray value images information concerning any defects present in the tire is obtained, and furthermore to an apparatus for the performance of the method.
The German patent publication 42 31 578 A1 discloses a method for the determination of the structural strength of tires, in which the tire is illuminated with coherent light, the radiation diffusely reflected back from the tire is divided up in a twin beam interferometer into two beam parts, in the twin beam interferometer one of the two beam parts is tilted in relation to the other beam part as a shearing operation, in the twin beam interferometer one of the two beam parts is phase shifted in steps, the radiation diffusely reflected by the object being tested and split up into two beam parts and then united again is returned through components presenting an image of the surface of the tire with a large aperture to an electronic image sensor system and the signals produced by the image sensor system are digitalized and further processed to give a modulo 2.sub..pi. image and the modulo 2.sub..pi. image is confirmed as an output gray value image.
In accordance with the German patent publication 195 02 073 A1 the above mentioned method is further developed to the extent that the output gray value image is partially differentiated, a second gray value image identical to the output gray value image is produced and it is geometrically displaced in the shearing direction in relation to the output gray value image and is altered by a gray value which is constant over the entire image area and the second gray value image manipulated in this manner is subtracted from the output gray value image and so that a resulting gray value image is produced, from which any defects in the tire may be seen.
When it is borne in mind that structural damage to a car tire and more particularly damage in the carcass including the belt, may substantially impair the safety of the vehicle, it will be seen to be desirable to perform examinations on tires at regular intervals.